Tape drives are used to store very large amounts of digital information on rolls of magnetic tape and are often used to backup information stored in computer systems. In a typical linear tape open (LTO) drive, magnetic tape is stored on a supply reel contained in a removable cartridge. Information on the tape, including servo information, is arranged in a multitude of parallel tracks that extend along the length of the tape. During operation, the tape is passed along a series of rollers, defining the tape path, to a non-removable take up reel in the tape drive. The tape passes in close proximity to an array of magnetic head elements that read and record information on the tape. The head elements must be accurately positioned over the desired tracks so information can be read or recorded without loss and without corrupting adjacent tracks. An actuator positions the head elements by moving the head containing the elements across the width of the tape. During coarse positioning, the actuator moves the head so that a read element is close enough to a desired track to read servo information. Subsequently, during fine positioning, the servo information is read from the track and sent to servo control circuitry, which then sends a signal to the actuator to move the head so that the elements are directly over the desired tracks and to follow the small lateral motion of the track as it passes by the head.
The capacity of a linear recording tape is determined, in part, by the number of tracks that can be read and recorded across the width of the tape. To reliably read and record all tracks, the head, tape and servo positioning system must achieve accurate head to tape alignment within system tolerances, including the dimensional stability of the tape. Magnetic tapes tend to shrink over the useful life of the tape. In addition, magnetic tapes shrink and expand in response to changes in temperature and humidity. Hence, the width of the tape can and usually does vary over time. That is to say, the tape is not dimensionally stable. As the number of tracks on a tape increase, the adverse effect of tape dimensional instability on head to tape alignment also increases.